The invention relates to an arrangement for moving a measuring head of a measuring device into and out of a process space, the arrangement comprising: a valve member to be attached to a wall of the process space; a measuring device to be attached to the valve member, the measuring device comprising a body part provided with an elongated portion which in turn has a measuring head at one end thereof; and a placement device arranged to seize the measuring device and to move it through the valve member into a first position in which the measuring head extends into the process space and, correspondingly, into a second position in which the measuring head is not in contact with the process space.
The above arrangements are currently extremely widely known in different fields of technology. Examples of such fields include food industry, wood processing industry, chemical industry and different research areas. The measuring devices used in these are often optical devices, one advantage of which is that they do not disturb the process. Another advantage is that the measuring operation is stable and adaptable to new process conditions. Moreover, optical measurement is technically reliable. One example of a typical application worth mentioning is the measurement of the properties of black liquor in wood processing industry.
Over the years different arrangements have been proposed in processing industry for moving the measuring head of a measuring device into and out of a process space. A basic idea in these arrangements is to provide equipment that allows the measuring device to be moved out of the process and back into it while the process is running. When such arrangements are used for moving the measuring device, the tip of the device carrying the measuring head passes through a valve member, such as a ball or a slide valve. The measuring device has an elongated arm portion sealed to the valve member mechanics. When the measuring device is outside the valve member, the valve member may be closed, the measuring device being thus safely taken out of the process. The fluid used in the process is often dangerous to the user, i.e. it may be both caustic and hot. In addition, it may be highly pressurized, which further increases the safety risk. One example of such process fluids is the black liquor already mentioned, which is used in pulp industry. The measuring head of the measuring device may be re-inserted into the process by pushing the arm portion through the above-mentioned valve member.
One example that can be mentioned among prior art solutions for moving the measuring head of a measuring device into and out of a process space is the solution described in patent publication FI 84400. This known solution is based on the use of at least two screw bars.
In principle the solution of FI 84400 functions perfectly satisfactorily. A drawback of this solution is, however, that if the sealing between the elongated arm portion of the measuring device and the valve member mechanics is not perfect, process fluid may splatter onto the operator and in the environment during the moving of the measuring device. Another drawback of the solution disclosed in FI 84400 arises from problems relating to the simultaneous rotation of the two screw bars, which render the structure complicated and increase the need for servicing.
It is an object of the invention to provide an arrangement that enables the drawbacks related to the prior art to be eliminated. This is achieved by the arrangement of the invention, characterized in that the placement device comprises a stop element configured to seize the measuring device and to form a sealed casing around the body part of the measuring device, and a housing part arranged to attach to the valve member and to form a sealed casing around the stop element, and in that the placement device is equipped with means for moving the stop element seizing the body part of the measuring device inside the housing part in such a manner that the measuring device moves between the first position and the second position.
An advantage of the arrangement of the invention is above all that any splattering and leakage of process fluid that may take place during the moving of the measuring device do not cause danger to the staff or the environment. This is because the arrangement of the invention forms a completely closed structure around the measuring device so that in case of leakage the process fluid remains inside the arrangement instead of splattering or leaking into the environment. Process fluid that has leaked into the arrangement can be led to a suitable place in a controlled manner, thus eliminating any situations hazardous to the staff and the environment. A further advantage of the invention is that it solves problems related to an implementation involving the use of two screw bars, for example.